Electron source and production method therefor

ABSTRACT

An electron source capable of suppressing consumption of an electron emission material is provide. The present invention provides an electron source including: an electron emission material; and, an electron emission-suppressing material covering a side surface of the electron emission material, wherein a work function of the electron emission-suppressing material is higher than that of the electron emission material, and a thermal emissivity of the electron emission-suppressing material is lower than that of the electron emission material.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.16/317,640 filed Jan. 14, 2019, the contents of which are incorporatedherein in their entirety.

TECHNICAL FIELD

The present invention relates to an electron source used in an electronmicroscope, an electron beam lithography apparatus, a semiconductorphotomask plotting apparatus and the like using an electron beam, and aproduction method thereof.

BACKGROUND ART

Patent documents 1 to 6 and non-patent document 1 disclose a techniquefor limiting a electron-emitting region by covering a periphery of anelectron emission material with graphite material in an electron source,thereby enabling enhancement of luminance and suppression of surpluscurrent.

PRIOR ART DOCUMENTS Non-Patent Document

-   [Non-Patent Document 1] Electron Optical Systems (pp. 163-170) SEM    Inc., AMF O'Hare (Chicago), Ill. 60666-0507, U.S. Pat. S. A.

Patent Documents

-   [Patent Document 1] JP2005-228741A-   [Patent Document 2] JP2005-190758A-   [Patent Document 3] JP2000-173900A-   [Patent Document 4] JP2009-152645A-   [Patent Document 5] JP2012-069364A-   [Patent Document 6] JP2014-075336A

SUMMARY OF THE INVENTION Problems to be Resolved by the Invention

When the present inventors investigated the techniques described in theabove documents, it was found that, in some conditions, consumption ofthe electron emission material could not be sufficiently suppressedbased on the techniques described in the above documents.

The present invention has been made in view of such circumstances andprovides an electron source capable of suppressing consumption of anelectron emission material.

Means of Solving the Problems

That is, the present invention adopts the following means in order tosolve the above problem.

(1) An electron source including:

an electron emission material; and,

an electron emission-suppressing material covering a side surface of theelectron emission material,

wherein a work function of the electron emission-suppressing material ishigher than that of the electron emission material, and

a thermal emissivity of the electron emission-suppressing material islower than that of the electron emission material.

(2) The electron source according to (1), wherein the electron emissionmaterial includes at least one selected from the group consisting oflanthanum boride, cerium boride and iridium cerium.

(3) The electron source according to (1) or (2), wherein the sidesurface of the electron emission material has a (100) crystal plane atan outer peripheral portion thereof.

(4) The electron source according to any one of (1) to (3), wherein theelectron emission-suppressing material includes at least one selectedfrom the group consisting of metallic tantalum, metallic titanium,metallic zirconium, metallic tungsten, metallic molybdenum, metallicrhenium, tantalum carbide, titanium carbide and zirconium carbide.(5) The electron source according to any one of (1) to (4), wherein anend surface of the electron emission material is on the same plane as anend surface of the electron emission-suppressing material, and a normalto the plane is in a direction of emission of electrons.(6) The electron source according to any one of (1) to (5), whereinshape of the electron emission-suppressing material is a thin film.(7) The electron source according to (6), wherein the thin film has athickness of 0.1 to 2 μm.(8) The electron source according to any one of (1) to (7), furtherincluding a support member around the electron emission-suppressingmaterial.(9) The electron source according to (8), wherein the support member isclosely attached to the electron emission-suppressing material.(10) The electron source according to (8) or (9), wherein the supportmember is made of graphite.(11) A method for manufacturing an electron source according to any oneof (1) to (10), including an applying step and a solidifying step,

wherein, in the applying step, paste containing an electronemission-suppressing material is applied to a side surface of anelectron emission material, and

in the solidifying step, the paste is solidified.

(12) The method according to (11), further including an inserting stepbetween the applying step and the solidifying step, wherein, in theinserting step, the electron emission material having the applied pasteis inserted into an opening provided in a support member.

Effect of the Invention

According to the present invention, the consumption of the electronemission material can be suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a perspective view of an electron source 1 according toone embodiment of the present invention.

FIG. 1B shows a cross-sectional view of the electron source 1 accordingto one embodiment of the present invention passing through the center ofthe electron source 1.

FIG. 2A shows a perspective view of an electron emission material 3 inthe manufacturing process of the electron source 1.

FIG. 2B is a perspective view showing a state where paste 4 p containingan electron emission-suppressing material 4 is applied to a side surface3 b of the electron emission material 3 in the manufacturing process ofthe electron source 1.

FIG. 2C is a perspective view showing a state where the electronemission material 3 having the applied paste 4 p in the manufacturingprocess of the electron source 1 is inserted into an opening 5 d of asupport member 5.

FIG. 2D is a perspective view showing a state where a thin film of theelectron emission-suppressing material 4 formed by solidifying the paste4 p covers the electron emission material 3 in the manufacturing processof the electron source 1.

FIG. 3A shows a perspective view of an electron source 1 with anelectron emission material 3 having a quadrangular prism shape.

FIG. 3B is a cross-sectional view of the electron source 1 with theelectron emission material 3 having the quadrangular prism shape passingthrough the center of the electron source 1.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, an example of the present invention will be described withreference to the drawings, but the present invention is not limitedthereto.

1. Configuration of Electron Source 1

As shown in FIGS. 1A, 1B to 2A to 2D, an electron source 1 according toan embodiment of the present invention includes an electron emissionmaterial 3 and an electron emission-suppressing material 4 covering aside surface 3 b of the electron emission material 3. A work function ofthe electron emission-suppressing material 4 is higher than that of theelectron emission material 3. A thermal emissivity of the electronemission-suppressing material 4 is lower than that of the electronemission material 3. Preferably, a support member 5 is provided aroundthe electron emission-suppressing material 4.

The electron source 1 can be used after heating by a heater.Configuration of the heater is not limited as long as it can heat theelectron source 1. The heater is, for example, a graphite heater or atungsten heater. Electrons are emitted from an end surface (electronemission portion) 3 a of the electron emission material 3 by heating theelectron source 1 with the heater in a state where a high electric fieldis applied under vacuum.

Electron Emission Material 3

The electron emission material 3 releases the electrons by heating.Examples of the electron emission material 3 include rare earth boridessuch as lanthanum boride (LaB₆), cerium boride (CeB₆) and iridium cerium(Ir₅Ce). The work function and thermal emissivity of lanthanum borideare 2.8 eV and 0.77, respectively. The work function and thermalemissivity of cerium boride are 2.8 eV and 0.76, respectively. The workfunction and thermal emissivity of iridium cerium are 2.6 eV and 0.45,respectively.

In a case where the rare earth boride is used, it is preferable that theelectron emission material 3 is a single crystal processed so that the<100> orientation, which has a low work function and is easy to emit theelectrons, coincides with a direction of electron emission.

The electron emission material 3 nay be processed into a desired shapeby electric discharge machining or the like. The shape of the electronemission material 3 is not particularly limited, and may be, forexample, a columnar shape as shown in FIG. 1A or a quadrangular prismshape as shown in FIG. 3A. Length of the electron emission material 3 ispreferably 0.2 to 3 mm, more preferably 0.5 to 1.5 mm, and still morepreferably about 1 mm. In a case where the electron emission material 3has the columnar shape, its diameter is preferably 0.02 to 0.3 mm, morepreferably 0.05 to 0.15 mm, and still more preferably about 0.1 mm. In acase where the electron emission material 3 has the quadrangular prismshape, one side thereof is preferably 0.02 to 0.3 mm, more preferably0.05 to 0.15 mm, and still more preferably about 0.1 mm. When the lengthof the electron emission material 3 is less than 0.2 mm, handlingbecomes worse, and when it is greater than 3 mm, it becomes difficult toraise temperature.

In any of the shapes, evaporation of the electron emission material 3from the side surface 3 b of the electron emission material 3 issuppressed by covering the side surface 3 b of the electron emissionmaterial 3 with the electron emission-suppressing material 4. It ispreferable that the electron emission-suppressing material 4 covers theentire circumference of the side surface 3 b of the electron emissionmaterial 3.

Crystal orientation of the side surface 3 b of the electron emissionmaterial 3 is not limited in general. However, since interatomic bondson a lower index crystal plane are denser, it is considered thatevaporation rate becomes slower. Thus, it is preferable that theelectron emission material 3 has (100) crystal planes on the outerperipheral portion thereof by processing the electron emission material3 so as to have the quadrangular prism shape and setting crystalorientations of its four side faces to (100).

Electron Emission-Suppressing Material 4

The work function of the electron emission-suppressing material 4 ishigher than that of the electron emission material 3 and the thermalemissivity of the electron emission-suppressing material 4 is lower thanthat of the electron emission material 3. Since the work function of theelectron emission-suppressing material 4 is higher than that of theelectron emission material 3, emission of the electrons from the sidesurface 3 b of the electron emission material 3 is suppressed bycovering the side surface 3 b of the electron emission material 3 withthe electron emission-suppressing material 4. In addition, since thethermal emissivity of the electron emission-suppressing material 4 islower than that of the electron emission material 3, the temperaturerise of the electron emission material 3 is suppressed by covering theside surface 3 b of the electron emission material 3 with the electronemission-suppressing material 4. In addition, it is preferable that thethermal emissivity of the electron emission-suppressing material 4 islower than that of the support member 5. In such a case, the temperaturerise of the electron emission material 3 is further suppressed.

Work function difference determined by (the work function of theelectron emission-suppressing material 4)—(the work function of theelectron emission material 3) is preferably 1.0 eV or more, morepreferably 1.4 eV or more, still more preferably 1.6 eV or more. Thermalemissivity difference determined by (the thermal emissivity of theelectron emission material 3)—(the thermal emissivity of the electronemission-suppressing material 4) is preferably 0.05 or more, morepreferably 0.1 or more, further more preferably 0.2 or more, still morepreferably 0.3 or more. In a case where the work function difference isless than a certain value, emission current from the side surface cannotbe suppressed. Also, in a case where the emissivity is less than acertain value, evaporation-suppressing effect cannot be obtained.

The electron emission-suppressing material 4 preferably includes arefractory metal or a carbide thereof, more preferably includes at leastone selected from the group consisting of metallic tantalum, metallictitanium, metallic zirconium, metallic tungsten, metallic molybdenum,metallic rhenium, tantalum carbide, titanium carbide and zirconiumcarbide. In addition, the electron emission-suppressing material 4 mayinclude at least one of boron carbide and graphite. In addition, theelectron emission-suppressing material 4 may include at least one ofniobium, hafnium and vanadium.

The work function and thermal emissivity of each electronemission-suppressing material 4 are as shown in Table 1. The thermalemissivity of each material used in the present specification is quotedfrom “New Edition High Melting Compound Properties Handbook <Volume I>Japan-Soviet News Agency, 1994” and “Revised 2nd Edition Metal DataBook, Maruzen Co., Ltd., 1984.”

TABLE 1 Work function Thermal Table 1 [eV] emissivity Metallic tantalum4.3 0.45 Metallic titanium 4.3 0.75 Metallic zirconium 4.0 0.32 Metallictungsten 4.5 0.43 Metallic 4.4 0.37 molybdenum Metallic rhenium 4.9Unknown Tantalum carbide 3.2 0.65 Titanium carbide 3.3 0.94 Zirconcarbide 3.4 Unknown Boron carbide 5.2 0.85 Graphite 5.0 0.90 Metallicniobium 4.2 0.35 Metallic hafnium 3.9 Unknown Metallic vanadium 4.4 0.34

When the electron emission-suppressing material 4 is a mixture of aplurality of the materials, overall work function and thermal emissivityof the electron emission limiting-material 4 is determined by volumeratios of all of the materials constituting the electronemission-suppressing material 4. For example, in a case where theelectron emission-suppressing material 4 includes metallic tantalum andboron carbide and its volume ratio is 0.38:0.62, the overall workfunction is 4.3×0.38+5.2×0.62=4.86, and the overall thermal emissivityis 0.45×0.38+0.85×0.62=0.70. The volume ratio can be calculated fromweight ratio and density.

It is preferable that all of the materials constituting the electronemission-suppressing material 4 have a higher work function than theelectron emission material 3 and a lower thermal emissivity than theelectron emission material 3. On the other hand, some of the materialsconstituting the electron emission-suppressing material 4 may have alower work function than the electron emission material 3 or a higherthermal emissivity than the electron emission material 3. Even in such acase, it is essential that the overall work function of the electronemission-suppressing material 4 is higher than that of the electronemission material 3 and that the overall thermal emissivity of theelectron emission-suppressing material 4 is lower than that of theelectron emission material 3.

Shape of the electron emission-suppressing material 4 is preferably athin film, and a thickness thereof is preferably 0.1 to 2 μm, morepreferably 0.2 to 1 μm, and still more preferably 0.3 to 0.7 μm. In acase where the thickness is less than 0.1 μm or greater than 2 μm,problems such as peeling and poor adhesion may occur.

As shown in FIGS. 1A, 1B, 3A and 3B, the end surface 3 a of the electronemission material 3 is on the same plane as an end surface 4 a of theelectron emission-suppressing material 4, and a normal to the plane isin the direction of the electron emission.

Support Member 5

In the present embodiment, the support member 5 is provided around theelectron emission-suppressing material 4. Damage of the electronemission-suppressing material 4 is suppressed by providing the supportmember 5. The support member 5 is an arbitrary element and can beomitted if unnecessary.

As shown in FIG. 2B, the support member 5 has an opening 5 d, and thesupport member 5 can be provided around the electronemission-suppressing material 4 by inserting the electron emissionmaterial 3 covered with the electron emission-suppressing material 4into the opening 5 d. The support member 5 is preferably provided so asto be closely attached to the electron emission-suppressing material 4,and more preferably provided so as to be closely attached to the entirecircumference of the electron emission-suppressing material 4, resultingthat a gap between the electron emission material 3 and the supportmember 5 is filled with the electron emission-suppressing material 4. Inaddition, since heat generated by the electron emission material 3 ispromptly transmitted to the support member 5 through the electronemission-suppressing material 4, resulting in suppression of excessivetemperature rise.

The support member 5 is preferably made of the graphite. Even when thethickness of the electron emission-suppressing material 4 is locallythinned by the support member 5 made of the graphite, the emission ofthe electrons from the side surface 3 b of the electron emissionmaterial 3 is suppressed by the support member 5 because the graphitehas a higher work function. Further, even when the thickness of theelectron emission-suppressing material 4 is locally thinned, reaction ofthe electron emission material 3 with the support member 5 is suppressedbecause reactivity of the electron emission material 3 with the graphiteis very low.

The support member 5 includes a side surface 5 a, a tapered surface 5 b,and an end surface 5 c. The side surface 5 a is connected with the endsurface 5 c via the tapered surface 5 b, and the support member 5 taperstoward the end surface 5 c. The end surface 5 c is on the same plane asthe end surface 3 a and the end surface 4 a.

2. Method for Manufacturing Electron Source 1

Next, a method of manufacturing the electron source 1 will be described.The electron source 1 can be produced by covering the side surface 3 bof the electron emission material 3 with the electronemission-suppressing material 4. Examples of the methods include amethod of forming the thin film of the electron emission-suppressingmaterial 4 on the side surface 3 b by vapor deposition (CVD or PVD) anda method of applying paste 4 p containing the electronemission-suppressing material 4 to the side surface 3 b of the electronemission material 3, and after that, solidifying the paste 4 p. Thelatter method is excellent in that a manufacturing equipment is simple.The latter method will be described in detail below.

An example of a method for manufacturing the electron source 1 by pasteapplication includes an applying step, an inserting step, a solidifyingstep, and a polishing step. In a case where the support member 5 is notprovided, the inserting step is unnecessary. Also, the polishing stepcan be omitted.

Applying Step

As shown in FIGS. 2A to 2B, in the coating process, the paste 4 pincluding the electron emission-suppressing material 4 is applied to theside surface 3 b of the electron emission material 3. The paste 4 p maybe applied to the entire side surface 3 b or may be applied to a portionof the side surface 3 b other than a vicinity of an end part 3 c asshown in FIG. 2B. It is preferable that the paste 4 p is applied so asto have the thickness of the electron emission-suppressing material 4described above after the solidifying step. In a case where the electronsource 1 includes the support member 5, it is preferable that the paste4 p is applied to have a thickness capable of filling a gap between aninner surface of the opening 5 d and an outer surface of the electronemission material 3.

The paste 4 p can be prepared by dispersing powder of the electronemission-suppressing material 4 in a dispersion medium. As thedispersion medium, water, organic solvent and the like can be used, andthe water is preferable.

The powder of the electron emission-suppressing material 4 may becomposed of only powder of the refractory metal such as titanium,zirconium, tantalum, niobium, hafnium, vanadium, tungsten, molybdenumand rhenium, which may contain powder of the graphite or powder ofceramic such as metal carbide (e.g., boron carbide). The powder ofceramic is preferably 10 parts by volume or more and 200 parts by volumeor less with respect to 100 parts by volume of metal powder as a solidcontent. In a case where blended amount of the ceramic powder is toolarge, bonding strength is lower. Conversely, in a case where theblended amount is too small, temporary adhesiveness before bonding ispoor, which causes problems in workability.

Inserting Step

As shown in FIGS. 2B to 2C, in the inserting step, the electron emissionmaterial 3 having the applied paste 4 p is inserted into the opening 5 dprovided in the support member 5.

The opening 5 d of the support member 5 can be formed by machining. Sizeof a cross section of the opening 5 d is larger than that of theelectron emission material 3. The size of the opening 5 d is, forexample, 0.15 mm in diameter×0.8 mm in depth.

When the electron emission material 3 is inserted into the opening 5 din a state where the paste 4 p is not applied, a gap is generatedbetween the outer surface of the electron emission material 3 and theopening 5 d, resulting in non-fixed state. On the other hand, when theelectron emission material 3 having the applied paste 4 p is insertedinto the opening 5 d, the gap is filled with the paste 4 p.

As shown in FIG. 2C, it is preferable that the end part 3 c of theelectron emission material 3 protrudes from the support member 5 in astate where the electron emission material 3 is inserted in the opening5 d.

Solidifying Step

Next, as shown in FIGS. 2C to 2D, in a state where the electron emissionmaterial 3 having the applied paste 4 p is inserted in the opening 5 d,the paste 4 p is solidified by performing a vacuum heat treatment,resulting that the thin film of the electron emission-suppressingmaterial 4 is coated on the electron emission material 3. Further, bysolidifying the paste 4 p, the electron emission material 3 and theelectron emission-suppressing material 4 can be fixed to the supportmember 5.

Polishing Step

Next, the end part 3 c of the electron emission material 3 is polishedusing a polishing member such as polishing paper or lapping film. Bydoing so, the end surface 3 a of the electron emission material 3, theend surface 4 a of the electron emission-suppressing material 4, and theend surface 5 c of the support member 5 are on the same plane, and theelectron source 1 shown in FIG. 1A is obtained.

EXAMPLES 1. Manufacture of Electron Source 1 Example 1

As an electron emission material 3, a cylindrical rod having a shape ofa diameter of 0.1 mm×a height of 1 mm with a longitudinal axis of <100>direction was produced from a single crystal of lanthanum boride byelectric discharge machining. It was difficult to limit crystalorientation of side surface thereof, but it was about 45 degrees off(100).

Next, high-purity graphite having a quadrangular prism shape of 0.7mm×0.7 mm×1.2 mm was prepared and then an end part thereof was sharpenedby machining to prepare a support member 5. An opening 5 d having adiameter of 0.15 mm×a depth of 0.8 mm was formed in the longitudinaldirection of the support member by machining.

Paste 4 p obtained by dissolving tantalum powder as an electronemission-suppressing material 4 with water was applied to a side surface3 b of the electron emission material 3. Then, the electron emissionmaterial 3 having the applied paste 4 p was inserted into the opening 5d of the support member 5.

Heater blocks were prepared by cutting pyrolytic graphite into a size of0.7 mm×0.7 mm×0.7 mm. Then, the support member 5 sandwiched between theheater blocks using a support pole while applying pressure wasassembled.

In such assembled state, current was applied under vacuum of 10⁻⁵ Pa,and held at 1600° C. for 2 minutes to solidify paste 4 p. Thereby, astructure in which the electron emission material 3 covered with theelectron emission-suppressing material 4 was inserted in the opening 5 dwas obtained. In this structure, as shown in FIG. 2D, the end part 3 cof the electron emission material 3 protruded from the support member 5.

Next, the support member 5 was detached from the support pole, and theend part 3 c of the electron emission material 3 was polished withabrasive paper so that an end surface 3 a of the electron emissionmaterial 3, an end surface 4 a of the electron emission-suppressingmaterial 4, and an end surface 5 c of the support member 5 were on thesame plane. Thereby, an electron source 1 in a state where the electronemission material 3 is coaxially surrounded by the electronemission-suppressing material 4 and the support member 5 was obtained.

Examples 2 to 3

Each of electron source 1 was produced in the same manner as in Example1 except that powder obtained by mixing tantalum powder and boroncarbide (product name: Denka Boron Carbide #1000) at a volume ratioshown in Table 2 was used as the electron emission-suppressing material4 instead of the tantalum powder and that the temperature at which thepaste 4 p was solidified was changed to 1550° C.

Example 4

An electron source 1 was obtained in the same manner as in Example 1except that cerium boride was used as the electron emission material 3instead of lanthanum boride.

Example 5

As an electron emission material 3, a side surface of a single crystalof lanthanum boride with a longitudinal axis of <100> direction wassubjected to electric discharge machining to produce a quadrangularprism shaped rod having a height of 0.1 mm, a width of 0.1 mm, and alength of 1 mm. The electric discharge machining was performed so that a(100) crystal planes were at an outer peripheral portion of the electronemission material.

Thereafter, a thin film of tantalum having a thickness of about 0.5 μmwas formed on the surface of the electron emission material 3 by CVDmethod. Next, colloidal carbon paste was applied to a side surface 3 bof the electron emission material 3, and then an electron source 1 wasproduced under the same conditions as in Example 2.

Examples 6 to 7

Each of electron sources 1 was produced in the same manner as in Example5 except that a metal of a type shown in Table 2 was used instead oftantalum.

Comparative Example 1

An electron source 1 was produced by the same process as in Example 1except that only colloidal carbon was used as a paste.

Comparative Example 2

An electron source 1 was produced by the same process as in Example 4except that only colloidal carbon was used as a paste.

Comparative Example 3

An electron source 1 was produced by the same process as in Example 1except that powder obtained by mixing tantalum powder and boron carbideat a volume ratio shown in Table 2 was used as the electronemission-suppressing material 4 instead of tantalum powder.

2. Evaluation of Electron Source 1

Each of the electron source 1 was attached to the support pole andsandwiched between graphite heaters. In order to evaluate heatresistance, each of the electron source 1 was continuously heated fortwo weeks at 1550° C. being temperature at normal operation under vacuumof 10⁻⁵ Pa, and then taken out. Next, consumption state of an outerperipheral portion of each of the electron emission material 3 wasobserved from the end surface 3 a by a scanning type electronmicroscope, and then the remaining diameter of the end surface 3 a whichis the electron emission portion was measured. The results are shown inTable 2.

As shown in Table 2, in Examples 1 to 7, the remaining diameter of eachof the electron emission portion was larger than that of each ofComparative Examples 1 to 3. In Examples 1 to 7, consumption of theouter peripheral portion of each of the electron emission material 3 washardly observed, but in Comparative Examples 1 to 3, consumption of theouter peripheral portion of each of the electron emission material 3 wasconspicuous.

In all Examples and Comparative Examples, a work function of each of theelectron emission-suppressing material 4 was higher than that of therespective electron emission material 3. On the other hand, in Examples1 to 7, a thermal emissivity of each of the electronemission-suppressing material 4 was lower than that of the respectiveelectron emission material 3 whereas, in Comparative Examples 1 to 3, athermal emissivity of each of the electron emission-suppressing material4 was higher than that of the respective electron emission material 3.

The above results demonstrate that the consumption of the electronemission material 3 can be prevented by covering the side surface 3 b ofthe electron emission material 3 with the electron emission-suppressingmaterial 4 having the higher work function than the electron emissionmaterial 3 and having the lower thermal emissivity than the electronemission material 3.

TABLE 2 Heat resistance evaluation Electron emission material Electronemission-suppressing material Remaining Crystal Work diameter oforientation function Overall Thermal Work electron Work of side of eachwork emissivity Overall function Thermal emission function surfaceThermal Volume material function of each thermal difference emissivityportion Type [eV] [off (100)] emissivity Type ratio [eV] [eV] materialemissivity [eV] difference [μm] Example 1 LaB₆ 2.8 45 degrees 0.77 Ta1.00 4.3 4.3 0.45 0.45 1.50 0.32 70 2 LaB₆ 2.8 45 degrees 0.77 Ta 0.384.3 4.86 0.45 0.70 2.06 0.07 65 B₄C 0.62 5.2 0.85 3 LaB₆ 2.8 45 degrees0.77 TaC 0.53 3.2 4.14 0.65 0.74 1.34 0.03 60 B₄C 0.47 5.2 0.85 4 CeB₆2.8 45 degrees 0.76 Ta 1.00 4.3 4.3 0.45 0.45 1.50 0.31 70 5 LaB6 2.8  0degree 0.77 Ta 1.00 4.3 4.3 0.45 0.45 1.50 0.32 83 6 LaB6 2.8  0 degree0.77 Ti 1.00 4.3 4.3 0.75 0.75 1.50 0.02 66 7 LaB6 2.8  0 degree 0.77 W1.00 4.5 4.5 0.43 0.43 1.70 0.34 83 Compar- 1 LaB₆ 2.8 45 degrees 0.77 C1.00 5.0 5 0.90 0.90 2.20 −0.13 40 ative 2 CeB₆ 2.8 45 degrees 0.76 C1.00 5.0 5 0.90 0.90 2.20 −0.14 40 Example 3 LaB⁶ 2.8 45 degrees 0.77 Ta0.13 4.3 5.08 0.45 0.80 2.28 −0.03 48 B₄C 0.87 5.2 0.85

EXPLANATION OF SIGNS

-   1: Electron source-   3: Electron emission material-   3 a: End surface-   3 b: Side surface-   3 c: End part-   4: Electron emission-suppressing material-   4 a: End surface-   4 p: Paste-   5: Support member-   5 a: Side surface-   5 b: Tapered surface-   5 c: End surface-   5 d: Opening

What is claimed is:
 1. An electron source comprising: an electronemission material; and, an electron emission-suppressing materialcovering a side surface of the electron emission material, wherein awork function of the electron emission-suppressing material is higherthan that of the electron emission material, and the side surface of theelectron emission material has a (100) crystal plane at an outerperipheral portion thereof.
 2. The electron source according to claim 1,wherein the electron emission material comprises at least one selectedfrom the group consisting of lanthanum boride, cerium boride and iridiumcerium.
 3. The electron source according to claim 1, wherein theelectron emission-suppressing material comprises at least one selectedfrom the group consisting of metallic tantalum, metallic titanium,metallic zirconium, metallic tungsten, metallic molybdenum, metallicrhenium, tantalum carbide, titanium carbide and zirconium carbide. 4.The electron source according to claim 1, wherein an end surface of theelectron emission material is on the same plane as an end surface of theelectron emission-suppressing material.
 5. The electron source accordingto claim 4, wherein a normal to the plane is in a direction of emissionof electrons.
 6. The electron source according to claim 1, wherein shapeof the electron emission-suppressing material is a thin film.
 7. Theelectron source according to claim 6, wherein the thin film has athickness of 0.1 to 2 μm.
 8. The electron source according to claim 1,further comprising a support member around the electronemission-suppressing material.
 9. The electron source according to claim8, wherein the support member is closely attached to the electronemission-suppressing material.
 10. The electron source according toclaim 8, wherein the support member is made of graphite.
 11. A methodfor manufacturing an electron source according to claim 1, comprising anapplying step and a solidifying step, wherein, in the applying step,paste containing electron emission-suppressing material is applied to aside surface of an electron emission material, and in the solidifyingstep, the paste is solidified.
 12. The method according to claim 11,further comprising an inserting step between the applying step and thesolidifying step, wherein, in the inserting step, the electron emissionmaterial having the applied paste is inserted into an opening providedin a support member.